So what had happened was simply that the certificate of the access points had expired due to their age.
If I set the time of the WLC back to September 25th of 2016, the access points would join prefectly, furthermore validating the certificate problem.

Since the softare releases 7.0.252.0, 7.4.140.0, and 8.0.120.0, the WLC can be configured to ignore expired certificates and let access points join regardless their expiration date.

The commands are as follows
For Version 7.0.252.0, use this command: config ap lifetime-check {mic|ssc} enable
For Versions 7.4.140.0 and later, use this command: config ap cert-expiry-ignore {mic|ssc} enable